The following description will make specific reference to the use of the catalyst disclosed herein in the transalkylation of PIPBs with benzene to afford cumene, but it is to be recognized that this is done solely for the purpose of clarity and simplicity of exposition. Frequent reference will be made herein to the broader scope of this application for emphasis.
Cumene is a major article of commerce, with one of its principal uses being a source of phenol and acetone via its air oxidation and a subsequent acid-catalyzed decomposition of the intermediate hydroperoxide.
Because of the importance of both phenol and acetone as commodity chemicals, there has been much emphasis on the preparation of cumene and the literature is replete with processes for its manufacture. The most common and perhaps the most direct method of preparing cumene is the alkylation of benzene with propylene, especially using an acid catalyst.
Another common method of preparing cumene is the transalkylation of benzene with PIPB, particularly di-isopropylbenzene (DIPB) and tri-isopropylbenzene (TIPB), especially using an acid catalyst. Any commercially feasible transalkylation process must satisfy the requirements of a high conversion of polyalkylated aromatics and a high selectivity to monoalkylated products.
The predominant orientation of the reaction of benzene with PIPB resulting in cumene corresponds to Markownikoff addition of the propyl group. However, a small but very significant amount of the reaction occurs via anti-Markownikoff addition to afford n-propylbenzene (NPB). The significance of NPB formation is that it interferes with the oxidation of cumene to phenol and acetone, and consequently cumene used for oxidation must be quite pure with respect to NPB content.
Because cumene and NPB are difficult to separate by conventional means (e.g. distillation), the production of cumene via the transalkylation of benzene with PIPB must be carried out with a minimal amount of NPB production. One important factor to take into consideration is that the use of an acid catalyst for the transalkylation results in increased NPB formation with increasing temperature. Thus, to minimize NPB formation, the transalkylation should be carried out at as low a temperature as possible.
Since DIPB and TIPB are not only the common feeds for the transalkylation of benzene with PIPBs but also the common byproducts of the alkylation of benzene with propylene when forming cumene, transalkylation is commonly practiced in combination with alkylation to minimize the production of less valuable byproducts and to produce additional cumene. In such a combination process, the cumene produced by both alkylation and transalkylation is typically recovered in a single product stream. Since NPB is also formed in alkylation and the amount of NPB formation in alkylation increases with increasing temperature, the NPB production in both alkylation and transalkylation must be managed relative to one another so that the cumene product stream is relatively free of NPB.
What is needed is an optimum transalkylation catalyst for, e.g., cumene production, with sufficient activity to effect transalkylation at acceptable reaction rates at temperatures sufficiently low to avoid unacceptable NPB formation. Because Y zeolites show substantially greater activity than many other zeolites, they have received close scrutiny as a catalyst in aromatic transalkylation. However, a problem exists in that Y zeolites effect transalkylation at unacceptably low rates at the low temperatures desired to minimize NPB formation.
Therefore, in order for a commercial process based on Y zeolites to become a reality, it is necessary to increase catalyst activity—i.e., increase the rate of cumene production at a given, lower temperature.